The invention relates to the field of optical amplifiers, and in particular to a solid-state optical amplifier intended for on-chip purposes.
Optical amplifiers are an essential component of optical circuits. As far as optical fiber communication is concerned, the well-developed Er-doped fiber amplifier (EDFA) technology is the platform for signal multiplexing and de-multiplexing in wavelength division multiplexing (WDM). Erbium can be pre-excited by 0.98 or 1.483 μm lasers to amplify incoming light signal by 30 dB without using any other devices. This is one of the many reasons for the widespread application of EDFA technology. However, the size of an EDFA is large and usually has a length of 10 m or longer. For this reason, it is too large to implement the device as an on-chip circuitry.
The semiconductor optical amplifier (SOA) is another possible technology platform that has the added advantage of enabling on-chip integration. In SOA operation, laser diodes are kept in a state with injection slightly lower then lasing. With incoming light, stimulated emission occurs from the laser diode. The bandwidth is determined by thermal fluctuation of the Fermi-Dirac distribution function at room temperature and is similar to the EDFA. The integration of SOA, however, is limited by the following two reasons. First, the SOA needs a high level of carrier injection to achieve population inversion of carriers. Such a current injection results in improper device operation, the so-called heat penalty. Second, SOAs are usually made of III-V semiconductors with direct band gaps, which are not easily implemented in current CMOS Si LSICs technology. Thus, a new type of optical amplifier technology is needed for on-chip integration of electronic and photonic circuits.